This invention relates to the field of display systems, more particularly to sequential color display systems that use a white light source in combination with a sequential filter such as a color wheel to produce a full color image.
Many projection display systems use a single light modulator in combination with a white light source to produce a full color image. In order to produce a full color image, the white light source is filtered sequentially to produce a primary colored light beam that changes over time. Typically, a color wheel is used to allow a series of primary colored filters to be spun through the white light beam in rapid succession. As each filter passes through the light beam, the light beam becomes a primary color beam with the active primary color determined by the which portion of the color wheel is passing through the optical path.
During each primary color period, data for the appropriate color is provided to a spatial light modulator to enable the modulator to create a series of single color images. If the single color images are created in a rapid sequence, the viewer""s eye integrates the series of images to give the perception of viewing a single full-color image.
Because the data that must be written to the modulator depends on the position of the color wheel, the position of the color wheel is tightly controlled to synchronize the color wheel with the remainder of the display system. The transition period between adjacent color filtersxe2x80x94typically called a spoke periodxe2x80x94requires turning the modulator off to ensure only pure primary colored light is used to create each of the three primary colored image. Uncertainties and errors in the position or speed of the color wheel force the display system controller to lengthen the spoke periods to ensure only primary colored light is incident the modulator at the appropriate time. Unfortunately, the accumulated off time associated with the lengthened spoke periods creates a substantial drop in projector efficiency.
An additional problem arises when the display system is used to display one of many available video signalsxe2x80x94for example off the air broadcasts or other consumer television applications. The various channels received by a display system may be broadcast at different frame rates. For example, in Hong Kong both PAL (50 Hz) and NTSC (60 Hz) broadcasts are available at the same time.
Cinematic source material creates a similar problem. Movies originally recorded on film typically were filmed at a 24 Hz frame rate. When a movie is broadcast on television, it is typically converted to a 60 Hz frame rate through a 3:2 frame rate conversion process. Display systems can detect the 3:2 frame rate conversion and reconvert the received signals to the original 24 Hz frame rate. Additionally, Modem ATV or HDTV broadcast standards allow for 24 Hz broadcast frame rates. While many display systems adapt to the 24 Hz frame rate, the frame rate can change abruptly when a commercial is broadcast. Likewise, the frame rate can change abruptly anytime a channel change occurs. The display system therefore must be very agile to adapt to the various frame rates that can be presented without warning.
Each time a signal changes, not only can the frame rate change, but the phase of the signalxe2x80x94alignment of the beginning of the framesxe2x80x94also changes. CRT-based displays handle these changes quite well, the new vertical retrace is detected and the new frame starts immediately while circuitry locks onto the new vertical and horizontal synchronization signals. Display systems that use color wheels, however, have a much more difficult time changing the frame rate and frame alignment. The speed of the color wheel must be altered to reposition of the color wheel relative to the received video signal. Because the color wheel has mass and is spinning relatively quickly, a high-torque motor is needed to make rapid speed and position changes. The high torque motor is expensive and can be noisy as it changes speed.
A method and system for controlling a color wheel with a low torque motor while providing a rapid response to frame rate and alignment changes is needed. The new method and system should reduce the time required to resynchronize with a new video signal, limit the noise generated by the color wheel, and be relatively inexpensive to implement.
Objects and advantages will be obvious, and will in part appear hereinafter and will be accomplished by the present invention which provides a method and system for a common color wheel speed system. According to one embodiment of the disclosed system, a display system is provided. The display system comprises a light source for producing a beam of white light along a first light path, a filter wheel on the first light path for filtering the beam of white light, the filter wheel having at least one set of primary colored filters thereon, a motor connected to the filter wheel for spinning the filter wheel at a nominal speed, a spatial light modulator on the first light path for receiving the filtered beam of light traveling along the first path and selectively modulating the filtered beam of light traveling along the first path to form an image, and a controller receiving an input video signal and providing image data decoded from the input video signal to the spatial light modulator, the input video signal having one of at least two formats and at least two nominal frame periods, the controller dividing the frame periods into sub-frames during which the image data is used by the spatial light modulator to form the image, the number of sub-frames dependent on the format of a received input video signal and the nominal speed of the color wheel.
According to one embodiment of the disclosed invention, the color wheel contains two sets of color filters and rotates at 150 Hz. NTSC signals are displayed using five sub-frames during the period in which the color wheel rotates 2.5 revolutions. PAL signals are displayed using six sub-frames during the period in which the color wheel rotates 3 revolutions. Alternate embodiments enable a common color wheel speed to be used for both film-based and NTSC formats.
According to one alternate embodiment, a nominal color wheel speed of 180 revolutions per second is used with a color wheel having two sets of filters. NTSC signals are displayed using six sub-frames during three rotations of the wheel. Film-based video signals, are displayed using fifteen sub-frames at a 24 Hz frame rate during 7.5 revolutions of the color wheel.
According to another alternate embodiment, a nominal color wheel speed of 120 revolutions per second is used with a color wheel having three sets of filters. NTSC signals are displayed using six sub-frames during two rotations of the wheel. Film-based video signals, are displayed using fifteen sub-frames at a 24 Hz frame rate during five revolutions of the color wheel.